1. Field of the Invention
The invention relates to a process for purifying aniline produced by gas-phase hydrogenation of nitrobenzene, by fractional condensation of the crude reaction product obtained in gaseous form so that at least two liquid process products, a partial condensate (PK) and a total condensate (TK), are obtained, wherein the condensation leading to the formation of PK is carried out at higher condensation than the temperature leading to the formation of TK, a product stream originating from PK and a product stream originating from TK are each passed separately from one another into a distillation column consisting of at least one stripping section and at least one rectifying section, wherein the product stream originating from PK is introduced into the lower section of the distillation column between the lowermost stripping section and the section which follows, and the product stream originating from TK is introduced into the top of the distillation column above the uppermost rectifying section, and the desired purified aniline is withdrawn in a side stream between the lowermost stripping section and the uppermost rectifying section of the distillation column.
2. Description of Related Art
Aromatic amines are important intermediates, which must be available inexpensively and in large amounts. Aniline, an aromatic amine which is of particular importance industrially, can be purified in an outstanding manner by the process according to the invention. Aniline is an important intermediate in the production of di- and poly-isocyanates of the diphenylmethane series (MDI) and is produced on an industrial scale generally by catalytic hydrogenation of nitrobenzene. Installations with very large capacities must be constructed for that purpose in order to be able to cover the enormous worldwide requirement. The hydrogenation of nitrobenzene is preferably carried out in the gas phase on stationary, heterogeneous supported catalysts, such as, for example, Pd on aluminium oxide or carbon supports, in fixed-bed reactors at an absolute pressure of from 2 to 50 bar and a temperature in the range from 250 to 500° C. under adiabatic conditions in gas recycle mode; see EP-A-0 696 573, EP-A-0 696 574 and EP-A-1 882 681. “Gas recycle mode” means that the non-condensable gases (that is to say substantially hydrogen not reacted during the hydrogenation and any added inert gases) contained in the crude reaction product, optionally with the exception of small amounts diverted off to keep constant further gaseous components of the recycle gas—such as, for example, ammonia formed on the catalyst by deamination reactions, are fed back into the reaction.
In the production of aniline, water and organic secondary products are also formed in addition to the target product. Amounts of unreacted nitrobenzene may additionally also be present, depending upon the production process. Such organic secondary components, as well as any unreacted nitrobenzene, must be separated off before the aniline is used further. The organic secondary components and any unreacted nitrobenzene can be divided into two groups: a) the group of the “low boilers”, that is to say compounds or azeotropically boiling mixtures of individual components having boiling points below that of aniline (b.p.=184° C.), and b) the group of the “high boilers”, that is to say compounds or azeotropically boiling mixtures of individual components having boiling points above that of aniline. Nitrobenzene (b.p.=211° C.) accordingly belongs to the group of the high boilers.
A crude product stream of a gas-phase hydrogenation installation of nitrobenzene accordingly generally comprises:                aniline,        process water (i.e. the sum of water formed in the reaction and water optionally added to the starting gas stream),        non-condensable gases (excess hydrogen—optionally containing gaseous impurities such as, for example, methane, carbon oxides—and optionally added inert gases, for example for improving the selectivity of added nitrogen, see EP-A-1 882 681, and optionally gaseous secondary products, for example ammonia from deamination reactions),        low boilers, and        high boilers (which can optionally also contain amounts of unreacted nitrobenzene).        
An example of the group of the low boilers is benzene (b.p.=80° C.), and an example of the group of the high boilers is diphenylamine (b.p.=302° C.). The aniline can easily be separated from those two impurities mentioned as examples because their boiling points are very different from that of aniline (ΔTs=104 K and 118 K). On the other hand, it is precisely the high boilers which, after condensation of the product, make it necessary to evaporate and condense the aniline again, so that their presence is particularly problematical.
A particular difficulty is in addition the separation of those secondary products whose boiling points are very similar to those of aniline, because the outlay in terms of distillation is considerable here. In this context, the separation of phenol (b.p.=182° C.) represents a major challenge for distillation technology, which is reflected in the use of long distillation columns with a large number of plates and high reflux ratios, with a correspondingly high outlay in terms of investment and energy.
Compounds having phenolic hydroxy groups, that is to say compounds that carry at least one hydroxy group (—OH) directly on an aromatic ring, can generally be problematical in the working-up of aniline. In addition to phenol, which has already been mentioned, particular mention may be made of the various aminophenols.
The purification of aniline is therefore not a trivial matter and has considerable industrial importance. Many approaches address in particular the mentioned problem in connection with compounds having phenolic hydroxy groups. The solution consists in converting the compounds having phenolic hydroxy groups into the corresponding salts by reaction with suitable bases, the salts, as non-volatile compounds, being substantially easier to separate off.
JP-A-49-035341, US-A-2005 080294, EP-A-1 845 079 and EP-A-2-028 176 accordingly disclose processes in which an aromatic amine is distilled in the presence of a base. In that procedure, problems of solids deposition, fouling and/or a pronounced rise in viscosity in the distillation must be prevented by complex and/or expensive measures.
As an alternative to the removal of compounds having phenolic hydroxy groups from aniline during the distillation, JP-A-08-295654 describes an extraction with dilute aqueous alkali hydroxide solution. Disadvantages of that process are the high NaOH consumption and the formation—as a result of the low concentration of the alkali hydroxide solutions—of very large amounts of waste water containing alkali phenolate.
EP-A-1 845 080 describes a process for the purification of aniline by extraction with aqueous alkali metal hydroxide solution having a concentration >0.7% by mass, wherein the concentration and temperature are so adjusted that the aqueous phase always represents the lower phase in the subsequent phase separation.
JP-A-2007217405 describes a process in which the phenol-containing aniline is brought into contact with aqueous alkali metal hydroxide solution at least twice, in such a manner that the concentration of alkali metal hydroxide in the aqueous phase is from 0.1% by mass to 0.7% by mass.
JP-A-2005 350388 relates very generally to the improvement of aniline working-up. A process is described in which a portion of the bottom product of the aniline distillation column is discharged therefrom and converted into the gas phase separately, that is to say in a second evaporator which is different from the actual evaporator of the column. The gas phase so obtained is fed back into the pure aniline column; high boiler fractions that cannot be evaporated are separated off. A disadvantage of that process is that low boilers and water must be separated off upstream of the actual aniline distillation column by an additional distillation in a dewatering column in a process that is complex in terms of apparatus.
None of the mentioned publications discusses how to reduce the proportion of aniline that must be evaporated and condensed again in a distillation process. If the aniline to be purified is from a gas-phase process, it even passes through two condensations according to the prior art: the reaction product obtained in gaseous form is first condensed as completely as possible, the aqueous phase is separated off, and the resulting organic phase is distilled, that is to say the desired product is (i) condensed, (ii) evaporated and (iii) condensed again, which is very expensive in terms of energy and apparatus and leads to considerable thermal stresses on the aniline.
Accordingly, it was an object of the present invention to provide a process for purifying aniline from gas-phase hydrogenations, in which only a minimal proportion of the aniline itself must be evaporated and condensed again and in which the separation of compounds having phenolic hydroxy groups is achieved as effectively as possible with minimal losses of valuable aniline.